1. Field of the Invention
The present invention relates to a mobile communication system and, more particularly, to controlling a network overload due to a machine type communication (MTC) data communication in an MTC service of a mobile communication system.
2. Related Art
In a 3rd generation partnership project (3GPP) which enacts technical specifications of a third generation mobile communication system, in order to cope with various forums and new technologies associated with a fourth generation mobile communication, a research on a long term evolution/evolved packet core (LTE/EPC) technology has been conducted since an end of 2004 in an effort to improve and optimize performance of a 3GPP technology. A study of the EPC focused on 3GPP SA WG2 is related to a network technology directed to determine a structure of a network, together with the LTE on 3GPP TSG RAN, and to support for mobility between heterogeneous networks, and is one of important recent 3GPP standardization issues. This is an effort to develop a 3GPP system into a system that supports various wireless access technologies based on an IP and a work has been progressed with a goal of an optimized packet based system which minimizes a transmission delay with an improved data transmission capability.
Hereinafter, technical terms used in the description of the present invention will be described.                An MTC feature is a feature for supporting a communication between machine type communication (MTC) devices or a communication between the MTC device and an MTC server, in which the communication is performed without a human intervention therebetween, unlike an existing person-to-person connection. For example, an        
MTC application includes a communication between a vending machine and a server, a point of service (POS) device and a server, and an electricity or water meter and a server. Here, a corresponding device is referred to as the MTC device. The MTC may be referred to as a machine-to-machine communication or device-to-device communication.                A tracking area (TA) refers to an area in which E-UTRAN provides a service and includes one or plural E-UTRAN cells.        A routing area (RA) refers to an area GERAN/UTRAN provides a service and includes one or plural GERAN/UTRAN cells.        A tracking area identity (TAI) list refers to a list of TAIs that identify the tracking that the UE can enter without performing a tracking area updating procedure. The TAIs in a TAI list assigned by an MME to a UE pertain to the same MME area. For a detailed description of the TAI list, a quotation to a disclosure of a standard document 3GPP TS 24.301 v9.1.0 is used.        A mobility management entity (MME) area is the part of the network served by an MME. An MME area consists of one or several tracking areas. All cells served by an eNodeB are included in an MME area. For a detailed description of the MME area, a quotation to a disclosure of a standard document 3GPP TS 23.002 v9.2.0 is used.        UMTS: Universal Mobile Telecommunication System, which is a 3G network.        EPS: Evolved Packet System, which is used to collectively refer to a core network (an evolved packet core), various access networks, and terminals that support the network. In addition, the EPS is an advanced form of the UMTS network.        NodeB: a base station of the UMTS network, which is installed outdoor and has coverage of a macro cell size.        eNodeB: a base station of the EPS network, which is installed outdoors and a service coverage of a micro cell size.        UE: User Equipment, which means a terminal device.        IMSI: International Mobile Subscriber Identity, which is a unique identification of a user that is solely assigned on an international level in a mobile communication network.        SIM card: Subscriber Identity Module include, which includes user or subscriber information such as IMSI.        UICC: Universal Integrated Circuit Card that is used as a SIM card.        MTC: Machine Type Communication means to the communication that occurs between devices without human intervention.        MTC device: a terminal (UE) such as, for example, a vending machine, probes, etc., which has a communication capability through the core network and performs a particular object.        MTC server: a server on a network which manages the MTC device and sends and receives a data. The MTC server may be located outside of the core network.        MTC application: the actual application using the MTC device and the MTC server, which includes, for example, remote meter reading, shipment movement tracking, etc.        MTC feature: a function or feature of a network for supporting the MTC applications. A portion of specific features are required for a purpose of each application.        
For example, MTC monitoring (necessary for, e.g., remote meter reading in preparation for lost equipment), low mobility (in a case of a vending machine, movement hardly exists), etc. may be required.                RAN: Radio Access Network, which is used to collectively refer to a 3GPP wireless access such as RNC, NodeB, and eNodeB.        HLR (home location register)/HSS (home subscriber server): database (DB) indicating the subscriber information within the 3GPP network.        RANAP: An abbreviation for Radio Access Network Application Part, which means an interface between RAN and a node (MME/SGSN/MSC) that is responsible for controlling the core network.        
Cell camping (or camp) on state refers to a state in which the terminal selects a cell after completing a cell selection process or a cell reselection process. For a detailed description thereof, a quotation to a disclosure of a standard document 3GPP TS 36.304 v9.1.0 is used.                ISR (idle mode signaling reduction) is a service for improving efficiency of a network resource by, for example, reducing signaling for registering a location when the terminal moves between different access networks such as the E-UTRAN and the UTRAN/GERAN.        ICS (IMS centralized services) provides a consistent service to an IMS regardless of the access network to which the terminal is attached (i.e., even if the terminal is attached to not only IP-CAN but also to CS domain). For a detailed description thereof, a quotation to a disclosure of a standard document 3GPP TS 23.292 v9.4.0 is used.        IMS (IP multimedia subsystem) is a system which provides a multimedia service on an IP basis.        
Attach refers to a connection of the terminal to a network node, and, in a broad sense, includes an attach generated with a handover.
Hereinafter, the present invention will be described with reference to the technical terms described above.
FIG. 1 is a conceptual view illustrating a 3GPP service model for supporting the MTC.
Although 3GPP standard GSM/UMTS/EPS define a communication through a PC network for supporting the MTC, the present invention describes a method which is also applicable in a CS network.
In current technical specifications, a definition of a network structure has been proposed with using an existing bearer of 3GPP. On the other hand, using a short message service (SMS) for a data exchange between the MTC device and the MTC server has been proposed as one of alternative solutions. Using the SMS is suggested by considering a fact that the MTC application is characterized to apply to a small amount of a digital data such as metering information or product information, and an existing SMS approach and an SMS approach based on the IMS may be supported. Referring to FIG. 1, MTCsms is a data exchange interface through the existing SMS approach, and MTCi is a data exchange interface for a 3GPP bearer service and the IMS.
Hereinafter, an overload control in the MTC service will be described.
Examples of an overload occurring in the MTC are as follows: when performing a wrong function of the MTC server or the MTC application; when an external event occurs in which a number of MTC devices are connected; and when a specific program is set to repeatedly operate at a specific time. In this case, operating the MTC application program in connection with the core network is realistically very difficult to implement. It is because core network nodes (SGSN/MME/PGW, etc.) are easily damaged due to traffic congested data.
Therefore, a method of controlling the overload that occurs in the conventional core network nodes are as follows.
1) The network node may reject a particular connection. To determine whether to reject such connection, an APN or MTC group may be used. Alternatively, the MTC device may be controlled to connect to the network node at only a certain predefined time. However, in case of a congested condition, by sending a back off time from the network node to the MTC device, the network node may provide the MTC device with information which enables the MTC device not to connect during that time.
2) There exists a method in which a location of TAU/RAU, i.e., a cell to which the MTC device belongs, is updated before the MTC device is used at the certain predefined time. However, this is for purpose of registering an exact location before use because the MTC device has not been used for a long time. In this case, the MTC device has been in an off state for a long time, and therefore, information of an access location needs to be updated, however, the above method cannot be a fundamental solution to the problem.
3) The MTC device randomizes an access start time at which the MTC device is connected to the network node within the certain predefined time. The purpose of this is to avoid a concentrated traffic at the certain predefined time. Such method may be triggered in the MTC server of the MTC device;
On the other hand, when the overload occurs at the MTC and a network condition is changed, an attach requested to the network by the MTC device may be rejected, thereby causing an unnecessary loss to the network resource. Hereinafter, an example of an attach procedure and the network overload which is caused when the network rejects an attach request of the MTC device will be described.
In a conventional mobile communication system which supports the MTC service, the attach procedure of the MTC device is as follows: 1) the MTC device (terminal) registers the subscriber information with the MTC server. In other words, a user of the MTC device may, for example, set or change an activation or deactivation state of each MTC feature though a method such as a web based registration. A result of setting the activation or deactivation state of each MTC feature is stored in the core network. 2) The network identifies the subscriber information at a time point when the terminal attaches to the network (or the core network node). In other words, at the time point when the MTC device attaches the network, the core network nodes (for example, MME) determine whether to accept a corresponding terminal (i.e., MTC device) in the network by bringing the subscriber information through an interaction with a subscription registration server (e.g., HSS). 3) On the other hand, when the attach request of the terminal (e.g., request for activating a specific MTC feature) cannot be supported according to the network condition such as an operator policy, the network determines to reject the attach request of the terminal